More than three years after Michael Jackson’s death, his youngest brother continues to raise questions about the validity of the pop superstar’s will.

On Twitter and cable TV, Randy Jackson has called the five-page document signed in 2002 a fake. The one place he hasn't made the claims is a courtroom, where legal experts say he faces almost insurmountable hurdles to invalidate the will and stiff odds against ousting the men who run the lucrative estate.

In a recent letter, Randy Jackson and three of his siblings called on Jackson’s estate executors to resign and renewed claims that the will is a fake.

The letter states the family was too overwhelmed after the singer's death to meaningfully challenge the will that gave only family matriarch Katherine Jackson and Michael's three children — Prince, 15, Paris, 14, and Blanket, 10 — a stake in the estate.

“At that time we couldn’t possibly fathom what is so obvious to us now: that the Will, without question, it's Fake, Flawed and Fraudulent,” the letter originally signed by Randy, Jermaine, Janet and Rebbie Jackson states.

Recently, Jermaine Jackson rescinded his support for the letter and said it never should have been made public.

The delayed challenge likely dooms any effort to invalidate the will. Even if it was thrown out, it would not alter the stake received by the King of Pop’s three children, experts say and an appeals court has noted.

Randy Jackson has since posted on Twitter that he believes the estate is trying to isolate his mother to the detriment of her health. “It is my fear and belief, that they are trying to take my mother’s life,” Randy Jackson wrote last month.

The estate has denied the accusations. “We are saddened that false and defamatory accusations grounded in stale Internet conspiracy theories are now being made by certain members of Michael’s family whom he chose to leave out of his will,” it wrote in a statement.

Jermaine Jackson said he still has concerns about the estate’s operations but realizes “the way to address such matters is through the proper channels and via a private dialogue, not public conflict.”

Almost from the moment it was filed, the will has been a topic of controversy for some members of the Jackson family. The pop superstar’s father Joe Jackson attempted to get a stipend from the estate, but like his children, he was excluded from any share.

Katherine Jackson explored the possibility of challenging the executors and was given permission by a judge but settled before a full hearing was held.

The document is straightforward and simple, and many key provisions of how Jackson's estate is constructed are set out in a trust. That document has never been publicly released.

Many of the misgivings stem from the will's final page, which bears the signatures of three witnesses who claim Michael Jackson signed the document on July 7, 2002, in Los Angeles. Jackson’s family points out that the singer was in New York on that day, a point the Rev. Al Sharpton recently bolstered by showing video of the “Thriller” singer appearing with him at an event in Harlem that day.

“I don’t think that kind of extrinsic fraud would be enough to overturn the order admitting the will to probate,” said Marshall Oldman, a probate attorney who represented Peter Falk’s wife in a conservatorship proceeding.

He said the only valid argument of Jackson’s siblings is that they did not receive proper notice that their brother's will had been accepted into probate. Superior Court Judge Mitchell Beckloff accepted the will in November 2009. Any challenge would have had to been filed within four months, Oldman said.

The California 2nd District Court of Appeal noted in an October 2010 ruling against the singer’s father that the period to challenge the will had already expired. Even if the will were thrown out, the court noted, California law would require the estate to benefit Michael Jackson’s children.

“I don’t see how you come in three years later, and say, ‘oh, by the way, the will’s a fraud, a forgery, because he wasn’t in LA when he was purported to be,’ ” said Howard Klein, a probate attorney for nearly 50 years and partner in the Los Angeles firm Feinberg Mindel Brandt Klein & Kline. “It’s something that should have been brought up a long time ago.”

Randy Jackson, in comments on Twitter and to Sharpton on his MSNBC show, has repeatedly accused the estate’s executors of criminal conduct. Both Klein and Oldman said even if the executors were charged with wrongdoing, it wouldn’t open the door for more of Jackson’s relatives to gain access to the estate.

Jackson’s children are deemed his heirs without the will, and a 1997 version lodged with the court but never publicly released also doesn’t name the singer’s siblings as beneficiaries of his estate.

Klein said even if Jackson or other siblings try to challenge the document, their bid would likely be rejected because it is too late. The judge could also rule, as he did against family patriarch Joe Jackson, that because he isn’t a beneficiary of the will, he isn’t entitled to contest it.

“It would be a tough sell,” Klein said of any effort by another Jackson relative to challenge the will now.

The executors recently informed a judge that there have been $475 million in gross earnings for the estate since Michael Jackson died in June 2009 from an overdose of the powerful anesthetic propofol. Jackson died with more than $500 million in debts, and the earnings have been used to repay many of the singer’s creditors and provide a spacious hilltop home for Katherine Jackson and the children along with private schooling, staff, security, vacations and other perks.

Katherine Jackson has requested and the estate is recommending approval of a nearly $35,000 a month increase in her stipend so she can retain her own attorney, accountant and homes in Indiana and Las Vegas, court filings show.